Vibrational energy harvester devices offer electrical power generation in environments that lack light, temperature differentials, and/or pressure differentials. Instead, vibrations, and or movements, e.g., emanating from a structural support, which can be in the form of either a vibration at a constant frequency, or an impulse vibration containing a multitude of frequencies can be scavenged (or harvested) to convert movement (e.g., vibrational energy) into electrical energy. One particular type of vibrational energy harvester utilizes resonant beams freely extending from a base as a cantilever that incorporate a piezoelectric material that generates electrical charge when strained during resonance of the beams caused by ambient vibrations (driving forces), such as that described in U.S. patent application Ser. No. 14/173,131 to Vaeth et al.
Improvements are needed in the energy harvesting capabilities of such devices in systems which receive multiple impulses. In particular, cantilever based piezoelectric vibrational energy harvesters include a resonator beam that has an inherent resonant frequency. The resonator beam may be excited to vibrate at the inherent resonant frequency by a short acceleration impulse. Additional impulses applied to the vibrational energy harvester may either enhance or suppress the motion of the resonator beam, depending on the timing of the subsequent impulses relative to the resonant frequency. If an additional impulse is applied in phase with the resonator beam motion, the amplitude of the motion is increased. If, however, the additional impulse is applied out of phase with the resonator beam motion, the amplitude of the motion will be decreased. Thus, the performance of the harvester is dependent upon the timing between the impulses applied to the system.
By way of example, the timing between impulses is particularly relevant to vibrational energy harvesters utilized in systems such as a tire pressure monitoring system (TPMS), where the harvester experiences impulses as the tire flexes during its rolling motion on the road. When a portion of the tread of the tire where the harvester is located contacts the road surface, that portion of the tire is forced into a short flat shape, which in turn results in a change in the acceleration profile for the harvester, which is attached to the perimeter of the tire. This change in the radial acceleration of the tire is explained in K. B. Singh et al., “Piezoelectric Vibration Energy Harvesting System With An Adaptive Frequency Tuning Mechanism For Intelligent Tires,” Mechantronics 22:970-88 (2012), which is hereby incorporated by reference in its entirety.
For the majority of the tire's rotational period, there is a relatively constant centripetal acceleration for a portion of the tire located at the perimeter of the tire. When that portion of the tire initially contacts the road surface, there is an initial increase in radial acceleration. The initial increase in acceleration is then followed by an abrupt drop in radial acceleration to zero. The abrupt drop to zero provides a first impulse to the vibrational energy harvester, exciting motion of the resonator beam. The radial acceleration then remains at zero during the time it takes for the portion of the tire to rotate through its contact with the road surface. Once the portion of the tire rotates through its contact with the road surface, there is an abrupt positive enhancement in the radial acceleration, followed by a settling back to an equilibrium radial acceleration. The abrupt rise in radial acceleration back to or near equilibrium provides a second impulse to the vibrational energy harvester system. The second impulse will either enhance or suppress the vibration of the resonator beam excited by the first impulse, depending on the temporal width between the first and second pulses. The rotational speed of the tire and the circumference (or diameter) of the tire determine the temporal width. The vibration of the resonator beam, and thus the amount of energy harvested, can vary greatly depending on the speed of the vehicle. Therefore, it would be desirable to develop a piezoelectric energy harvester that provides a more consistent source of electrical energy in a system that is submitted to multiple impulses, such as in a TPMS.
The present invention is directed to overcoming these and other deficiencies in the art.